Binaural inhibition is important in shaping the free-field frequency
selectivity of single neurons in the inferior colliculus.
Gooler, David M., Jinghua Xu, and Albert S. Feng.
Department of Molecular and Integrative Physiology, and the Beckman
Institute, University of Illinois at Urbana-Champaign, Urbana, IL 61801.APStracts 3:0138N, 1996.
SUMMARY AND CONCLUSIONS
1. We have previously shown that under free-field stimulation in the frontal
field, frequency selectivity of the majority of inferior colliculus (IC)
neurons became sharper when the loudspeaker was shifted to ipsilateral
azimuths. These results indicated that binaural inhibition may be responsible
for the direction-dependent sharpening of frequency selectivity. To test the
above hypothesis directly, we investigated the frequency selectivity of IC
neurons under several conditions: i) monaural stimulation using a semi-closed
acoustical stimulation system, ii) binaural stimulation dichotically also
using a semi-closed system, iii) free-field stimulation from different
azimuths, iv) free-field stimulation when the ipsilateral ear was monaurally
occluded (coated with a thick layer of petroleum jelly which effectively
attenuated acoustic input to this ear). 2. The binaural interaction pattern of
98 IC neurons of northern leopard frogs (Rana pipiens pipiens) were evaluated
of which there were 34 EE and 64 EO neurons. The majority of IC neurons (92 of
98) showed some degree of binaural inhibition (i.e., showing diminished
response when the ipsilateral and contralateral ears were stimulated
simultaneously) whether they were designated as EE or EO; these IC neurons
were thus classified as EE-I or EO-I. Neurons were classified as exhibiting
strong inhibition if the ILD function showed a pronounced response decrement,
i.e., a decrease of ³ 50% of the response to monaural stimulation of the
contralateral ear. Those neurons that showed smaller response decrements
(decrease was ³25% but < 50%) were designated as showing weak inhibition. Most
of these EE-I and EO-I neurons (n=68) showed strong binaural inhibition. 3. In
agreement with results from our earlier studies, frequency threshold curves
(FTCs) of IC neurons were altered by sound azimuth. Independent of binaural
interaction pattern, most IC neurons (59 of 98) showed a narrowing of the FTC
as sound direction was changed from contralateral 90[acute]i (c90[acute]i) to
ipsilateral 90[acute]i (i90[acute]i). IC neurons that exhibited the largest
direction-dependent changes in frequency selectivity were typically those that
displayed stronger binaural inhibition. Occlusion of the ipsilateral ear,
which reduced the strength of binaural inhibition by this ear, abolished
direction-dependent frequency selectivity. 4. FTCs of IC neurons that
exhibited little to moderate direction-dependent effects on frequency
selectivity were typically associated with neurons that displayed weak
binaural inhibition. Associated with this weak binaural inhibition, central
neural responses under monaural occlusion also displayed only small effects;
the FTCs were only slightly broader than those derived in the intact
condition, and as before, the experimental manipulation resulted in
abolishment of direction-dependent frequency selectivity. 5. In contrast to
most IC neurons which showed direction-dependent narrowing of the FTC, about
one-third (34 of 98) of IC neurons studied showed a broadening of the FTC when
sound direction was shifted to ipsilateral azimuths. Interestingly, for 90% of
these 34 neurons, monaural occlusion resulted in narrowing of the bandwidth at
each azimuth instead of broadening of the FTC bandwidth. We have evidence to
suggest that this direction-dependent broadening is actually a consequence of
a truncation or loss of the tip of the FTC derived at c90[acute]i which
results from strong binaural inhibition. 6. To compare the frequency threshold
tuning in response to monaural stimulation of each ear with free-field FTCs we
measured FTCs for each of the 34 EE neurons to independent contralateral and
ipsilateral stimulation. FTCs derived from ipsilateral monaural stimulation
were significantly narrower than those resulting from contralateral monaural
stimulation, independent of a neuron's direction-dependent changes in
frequency selectivity. These results demonstrated that the narrower excitatory
FTCs to ipsilateral monaural stimulation cannot be the primary contributor to
the sharpening of FTCs in some EE cells, and broadening in other EE cells, in
response to stimulation from ipsilateral azimuths in the free field. 7. These
results support our hypothesis that binaural interactions, particularly
binaural inhibition, play a role in the sharpening of frequency selectivity in
the frog IC.

Received 7 August 1995; accepted in final form 18 June 1996.
APS Manuscript Number J511-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 4 July 96